The lignocellulose material in the process of the present invention is mechanically defibrated to fiberpulp and may consist of wood, straw or bagasse, and will be generally referred to as wood or, disintegrated in form of pieces, as chips or raw chips.
The method of the present invention is applicable to all defibering processes in which the defibration of chips takes place in an atmosphere of saturated steam at temperatures exceeding 100.degree. C., usually between 130.degree. C. and 200.degree. C. and corresponding steam pressures of 294 kiloPascals-1569 kiloPascals. The method provides a considerable saving of steam relative to that at present consumed in order to carry through the following conventional cycle, namely, presteaming of incoming raw chips to 90.degree. C.-100.degree. C. with steam at atmospheric pressure, defibration of the presteamed chips after heating with steam to the defibration temperature, blowing out fiberpulp and steam to a cyclone under atmospheric pressure and use of the separated 100.degree. C. steam for presteaming of the raw chips. The pulp constitutes the basic material for the manufacture of fiberboard and is diluted and if necessary refined before forming into wet laps, which are pressed and/or dried to finished product. In particular, the present method is suitable when so-called disc refiners are used and provides a nearly 100% utilization of the heat developed by the defibration for increasing the temperature of the chips before entering the defibration zone.
Heretofore, the chips have been heated with fresh steam to the defibration temperature in a preheater directly connected to the defibrator, at which the heat generated in the defibration zone has only to a small extent been utilized for increasing the temperature of the chips entering the defibration zone.
As energy consumed for the defibration of the chips to a considerable extent, probably more than 80%, is transformed into heat, this means that at an energy consumption of 150 kwh per ton of pulp (dry) from wood chips with a dryness of 50% and the incoming chips preheated to 95.degree. C., developed heat is sufficient to increase the temperature of the chips to 170.degree. C. (defibration temperature) and, theoretically, no extra steam has to be added for increasing the temperature of the incoming chips to the defibration temperature. For carrying through the whole process, only such an amount of low pressure steam will be consumed as is required for increasing the temperature of the raw chips to 95.degree. C.
It is known, as, for example, from U.S. Pat. No. 4,012,279, to preheat the incoming raw chips to the defibration unit with steam, which is released at the discharge of the pulp at atmospheric pressure. In the defibration process carried out up to this point, however, liberated steam contains considerably more heat than that consumed for the preheating of the raw chips to 90.degree. C.-100.degree. C., and this represents the main source of heat energy loss in the process.
In order to attain the lowest possible heat consumption in the defibration process, the released steam should not contain more heat than is consumed for preheating the incoming raw chips to 90.degree.-100.degree. C. This implies, generally, that all heat generated in the defibration zone has to be used for heating the chips before reaching the defibration zone. The generated heat, however, is usually not sufficient to bring the chips to the defibration temperature and, therefore, addition of fresh steam is required.